Xerographic plate containing aluminum selenide barrier layer



A ril 28, 1970 M. LEVY ,9

XEROGRAPHIC PLATE CONTAINING ALUMINUM SELENIDE BARRIER LAYER Filed June21, 1966 VOLTAG E 250 TIME (SECONDS) VOLTAGE 250 TIME (SECONDS) INVENTOR.

MORTIMER LEVY A 7' TORNE Y United States Patent Office 3,508,918Patented Apr. 28, 1970 3,508,918 XEROGRAPHIC PLATE CONTAINING ALUMINUMSELENIDE BARRIER LAYER Mortimer Levy, Rochester, N.Y., assignor to XeroxCorporation, Rochester, N.Y., a corporation of New York Filed June 21,1966, Ser. No. 559,276 Int. Cl. G03g 5/02 US. CI. 96-15 5 ClaimsABSTRACT OF THE DISCLOSURE A xerographic plate having a conductivesubstrate coated with a photoconductive layer, preferably of vitreousselenium, and a barrier layer sandwiched between the conductivesubstrate and photoconductive layer, with the barrier layer comprisingaluminum selenide.

This invention relates in general to xerography and, in particular, toan improved barrier layer for xerographic plates.

In the art of xerography, a xerographic plate containing aphotoconductive insulating layer is first given a uniform electrostaticcharge in order to sensitize its surface. The plate is then exposed toan image of activating electromagnetic radiation such as light, X-ray,or the like which selectively dissipates the charge in the illuminatedareas of the photoconductive insulator while leaving behind a latentelectrostatic image in the nonilluminated areas. The latentelectrostatic image may be developed and made visible by depositingfinely divided electroscopic marking particles on the surface of thephotoconductive insulating layer. This concept was originally disclosedby Carlson in US. Patent 2,297,691, and is further amplified anddescribed by many related patents in the field.

The general requirements of a photoconductive insulating layer are thatthe member support an electrostatic charge for a given period of time,preferably a long time, and that the member will support this chargewith a relatively small degree of charge dissipation under dark roomconditions in the absence of activating radiation. Another requirementis that the photoconductive member must be comparatively highlyconductive on exposure to activating illumination or radiation so thatthe electrostatic charge is rapidly dissipated in the illuminated areasupon exposure. Another related, but distinctively different requirementis that the charge dissipation be largely complete upon full exposure sothat the charge on the depleted area will be a relatively low andpreferably substantially zero potential. A further requirement is thatthe above properties be substantially retained from one cycle ofoperation to the next; that is, that the charge retention,photoconductivity, and completeness of charge dissipation be functionsof the photosensitive member and its exposure rather than functions ofunrelated conditions such as the number of charging, exposing anddevelopment cycles which have been carried out on a givenphotoconductive member.

Conventionally, a xerographic plate normally comprises a conductive basemember generally characterized by the ability to conduct electricity forcharging or sensitization of the composite member and to accommodate therelease of the electric charge upon exposure of the member to activatingelectromagnetic radiation. Generally, this conductive member must have aspecific resistivity less than about ohm-cm., and usually less thanabout 10 ohm-cm. The conductive support should also have sufficientstructural strength to provide a mechanical support for thephotosensitive member thus making it readily adaptable for xerographicmachines suitable for commercial use. This base support may be of anysuitable material and in any particular shape. Typical materialscomprise metallic plates, conductively coated plastic or glass,conductive paper, etc. The conductive support member may be in anyconvenient shape, such as a plate or plane, cylindrical surface, belt,web or the like.

Overlaying the conductive support is a photoconductive insulating layerwhich comprises any suitable photoconductor. Typical photoconductorssuitable for such an application are those such as shown by Carlsonabove, and the many related patents in the field. The discovery of thephotoconductive insulating properties of vitreous selenium has resultedin this material becoming the standard in commercial xerography.

In general, the photoconductor layer must have a specific resistivitygreater than about 10 ohm-cm. in the absence of illumination, andpreferably at least 10 ohm-cm. This resistivity should drop at leastseveral orders in magnitude in the presence of activating radiation. Thelayer generally should support an electrical potential of at least aboutvolts in the absence of radiation and may vary in thickness from about10 to about 200 microns.

A plate of the above configuration, normally under dark room conditionsexhibits a reduction in potential or voltage leakage in the absence ofactivating radiation which is known as dark decay, and exhibits avariation in the electrical performance upon repetitive cycling whichvariation is known in the art as fatigue.

The problem of dark decay and fatigue is well-known to the art and hasbeen remedied by the incorporation of a barrier layer comprising a thindielectric layer only a fraction of the thickness of the photoconductivelayer, and interdisposed between the conductive substrate and thephotoconductive layer.

US. Patent 2,901,348, to Dessauer et al. contemplates such a barrierlayer which comprises a thin layer or film of aluminum oxide (A1 0 in athickness range of about 25 to 200 angstroms or an insulating resinlayer such as polystyrene layer in the order of 0.1 to 2 microns inthickness. The barrier layer functions so as to allow thephotoconductive layer to support a charge of high field strength with aminimum charge dissipation in the absence of illumination. Whenactivated by illumination, the photoconductive layer becomes conductive,thereby causing a migration of the appropriate charges through saidphotoconductive layer and the appropriate dissipation of charge in theradiation or illumination struck areas.

In making the aluminum oxide barrier layer disclosed by the aboveDessauer et al. patent, it is necessary that a special oxidizing step beemployed in order that the aluminum oxide layer be formed on thealuminum substrate. This step may conventionally involve immersing amirror finished aluminum plate into a hot aqueous solution oforthophosphoric and nitric acid until the plate is free from anydetectable aluminum oxide followed by washing the plate in distilledwater and baking for 1 hour at 250 F. This treatment results in a thinuniform layer of aluminum oxide on the plate in thickness of about 100angstroms. The plate is then coated with a layer of vitreous selenium byconventional techniques such as shown by US. Patents 2,753,278 to Bixbyet al., 2,962,376 to Schatfert, and the above mentioned Dessauer et al.patent.

The need, therefore, arises for a barrier layer which may be simply andefliciently manufactured and still have the advantages of low darkdischarge.

It is, therefore, an object of this invention to provide a new andimproved photosensitive element which overcomes the above noteddisadvantages.

It is another object of this invention to provide a system utilizing aphotoconductive member which yields constant photosensitive propertiesupon repeated cycling.

It is another object of this invention to provide a photoconductivemember having improved dark decay properties.

It is another object of this invention to provide a system utilizing aphotosensitive member having relatively constant xerographic andphysical properties over long range cycling usage.

The foregoing objects and others are accomplished in accordance withthis invention by providing a photoconductive member having highphotosensitivity, and relatively low dark decay which is accomplished byproviding a novel barrier or junction layer comprising aluminum selenide(Al Se According to this invention, it has been found that a xerographicplate formed by depositing selenium on a freshly evaporated aluminumsurface, without allowing an aluminum oxide layer to build up, resultsin the formation of a successful charge barrier layer equivalent to theproperties of barrier layers of relatively thick aluminum oxide layersand yet requiring no special oxide formation. The barrier layer shouldbe from about 10 to 200 angstroms in thickness. The barrier layer isthought to be formed through a chemical reaction between selenium andaluminum at the surface to form an aluminum selenide compound. Theinterface formed by this invention is effective in controlling the darkdecay of a selenium photoconductor.

As mentioned above, in plate fabrication using an aluminum substrate,said substrate is oxidized by heating in a humid atmosphere to build upan oxide layer sufficiently thick (about 50 angstroms )to reduce theflow of charge from the substrate metal into the selenium layer. It isproposed by this invention that a barrier layer be formed through achemical reaction of the evaporated selenium with an oxide free aluminumsurface. The formation of even a very thin (approximately 10 angstroms)layer of oxide on the aluminum would prevent a reaction of evaporatedselenium with aluminum and such a layer would be too thin to act as acharge barrier layer.

The plate is formed by depositing selenium on a freshly evaporatedaluminum surface. The evaporations are made sequentially withoutbreaking the vacuum at aboutltorr. These plates exhibit a low dark decayof about 5 percent in 30 seconds. However, if the aluminum layer wasexposed to room air for about /2 hour then vacuum coated with selenium,the plate exhibits a relatively high dark decay. It is, therefore,essential that the surface of the aluminum be substantially free fromany aluminum oxide formation. Even highly polished aluminum plates orcoatings which are cleaned and allowed to be exposed to room air forseveral minutes will be contaminated with a thin film of aluminum oxidewhich will prevent the formation of the aluminum selenide barrier layer.

The advantages of this method will become apparent upon consideration ofthe following disclosure of the invention; especially taken inconjunction with the following drawings wherein:

FIG. 1 of the drawing graphically illustrates the rate of dark decay fora selenium coated xerographic plate using an aluminum selenideinterference barrier layer as contemplated by this invention.

FIG. 2 illustrates graphically the dark decay rate when a very thinaluminum oxide film is allowed to form on the surface of the aluminumsubstrate.

In FIG. 1 the rate of dark decay for a xerographic plate containing analuminum selenide barrier layer interpositioned between a seleniumphotoconductive overcoating and a NESA substrate is shown. The plateexhibits the low dark decay rate of about 5 percent in 30 seconds.

FIG. 2 illustrates the decay rate when an aluminum layer on a NESAsubstrate is exposed to room air for about /2 hour and then vacuumcoated with selenium. It can be seen from the graph that the plateexhibits a relatively high dark decay rate as compared to the plate ofFIG. 1 which was treated Without breaking the vacuum. As contemplated,this invention is directed to forming a barrier layer consisting of analuminum selenide layer between the photoconductive vitreous seleniumand a lower conductive substrate. The substrate may be of any suitablematerial and in any suitable shape or form such as has already beenpreviously described. The substrate may be aluminum which is free fromaluminum oxide or it may consist of a thin film of aluminum deposited ona conductive substrate such as NESA, aluminum coated on an insulatingsubstrate, aluminum deposited on a conductively coated plastic, or aconductive metal such as stainless steel, brass, or the like. Thesubstrate to be coated is placed in a vacuum chamber evacuated to apressure of about 10* torr or better. A source of aluminum is heated toevaporate a thin aluminum film of about 50 to 200 angstroms onto thesubstrate if said substrate is not aluminum. Without breaking thevacuum, a source of selenium is then placed in position for evaporationonto the aluminum layer. The selenium is then evaporated over thealuminum layer under the same vacuum conditions to the desiredthickness. The vacuum chamber is then cooled to room temperature, thevacuum broken, and the plate removed from the vacuum chamber. This plateexhibits a low dark decay rate due to the aluminum selenide barrierlayer. The aluminum is evaporated at a temperature exceeding about 660C. The selenium is evaporated at a temperature exceeding about 217 C.

When an aluminum substrate is used, it should be carefully washed andcleaned as previously described, and then immediately placed in a vacuumchamber before any aluminum oxide is allowed to form on its surface.This plate is then treated by evaporating a selenium layer onto saidaluminum substrate. This also results in the formation of an aluminumselenide barrier layer at the interface of the aluminum substrate andthe selenium overcoating.

The following examples specifically define the present invention withrespect to a method of making an aluminum selenide barrier layer in axerographic plate. The parts and percentages in the disclosure are byweight unless otherwise indicated. The examples below are intended toillustrate the various preferred embodiments of the invention.

EXAMPLE I A NESA substrate is placed in a vacuum chamber evacuated to apressure of about 10* torr. A boat containing aluminum pellets ispositioned approximately 6 inches below the NESA substrate and heated byresistance elements to a temperature of about 680 C. During theevaporation procedure the substrate is maintained at a temperature ofabout 50 C. Evaporation is complete in about 20 minutes at which timethe aluminum layer is about 50 angstroms in thickness. Without breakingthe vacuum, a source of selenium pellets is placed in the chamber 4inches below the NESA substrate and maintained at a temperature of about250- C. for approximately 30 minutes to evaporate a 40 micron seleniumlayer onto the previously deposited aluminum layer. After 30 minutes thetemperature is decreased to room temperature, the vacuum broken, and thecoated NESA plate removed from the chamber. The resulting plate containsa thin aluminum selenide barrier layer approximately 50 angstroms inthickness at the interface of the aluminum and selenium layers whichfunctions to sufliciently re duce dark decay to a tolerable value.

EXAMPLE II The method of Example I is employed to evaporate the aluminumlayer on the NESA substrate. The aluminum coated NESA plate is thenexposed to room air for 30 minutes at which time a thin aluminum oxidelayer (about angstroms thick) has formed on the aluminum coating. Theplate is then returned to the vacuum chamher and overcoated withselenium as in Example I.

EXAMPLE III The dark decay of the plates of Example I and II are bothmeasured under dark room conditions. This is accomplished by exposingthe plates to a positive corona spray and surface charging the plates toan average plate potential of about 500 volts. The voltage loss (darkdecay) is then measured over a 30 second interval by means of anelectrometer. The rate of dark decay for the plates of Examples I and IIare shown in FIGS. 1 and 2, respectively.

EXAMPLE IV The plate of Example I is corona charged to a positivesurface potential of about 500 volts, exposed to a light and shadowimage, and developed by cascading a developer material containingcarrier and toner :particles over the surface of the plate. Thedeveloped toner image is then transferred to a sheet of paper andpermanently fixed by heat fusing. A high quality reproduction of theoriginal image is produced by this method.

EXAMPLES V-VI Two plates are made by the method of Example I. The NESAsubstrate is replaced with a brass substrate for one plate, and astainless steel substrate for the other plate. When tested for darkdecay, both of these plates exhibit a low rate of decay similar to thatshown in -FIG. 1.

EXAMPLE VII A xerographic plate is made by the method of Example Iexcept that the aluminum coating step is eliminated by employing analuminum substrate. The clean aluminum, oxide-free substrate is placedin the vacuum chamber and overcoated with selenium as in Example I. Theresulting plate exhibits low dark decay when charged and tested underdark room conditions.

Although specific components and proportions have been stated in theabove description of the preferred embodiments of this invention, othersuitable materials and procedures such as those listed above may be usedwith similar results. In addition, other materials may be added to theplates which synergize, enhance or otherwise modify their properties.

Other modifications and ramifications of the present invention wouldappear to those skilled in the art upon reading the disclosure. Theseare intended to be included within the scope of the invention.

What is claimed is:

1. A xerographic plate comprising:

(a) a conductive substrate having thereon (b) an overlayingphotoconductive layer comprising vitreous selenium, and

(c) a barrier layer disposed between said conductive substrate and saidphotoconductive layer, with said barrier layer consisting essentially ofaluminum selenide in a thickness of about 10 to 200 angstroms.

2. A method of forming a xerographic image comprising:

(a) uniformly electrostatically charging a xerographic plate whichincludes a conductive substrate having an overlaying photoconductivelayer of vitreous selenium and a barrier layer disposed between saidconductive substrate and said photoconductive layer, with said barrierlayer consisting essentially of aluminum selenide in a thickness ofabout 10 to 200 angstroms;

(b) exposing said photoconductive layer to a pattern of activatingelectromagnetic radiation to form a latent electrostatic image thereon,and

(c) developing said latent electrostatic image to make it visible.

3. An imaging method comprising:

(a) providing a xerographic plate comprising a photoconductive layer ofvitreous selenium overlaying a conductive substrate and a barrier layerconsisting essentially of aluminum selenide in a thickness of about 10to 200 angstroms disposed between said photoconductive layer and saidsubstrate;

(b) forming an electrostatic latent image on said plate;

and

(0) developing said image with electroscopic marking particles to form avisible image.

4. A method of forming an electrostatic latent image which comprises:

(a) providing a xerographic plate comprising a photoconductive layer ofvitreous selenium overlaying a conductive substrate and a barrier layerconsisting essentially of aluminum selenide in a thickness of about 10to 200 angstroms disposed between said photoconductive layer and saidsubstrate;

(b) substantially uniformly electrostatically charging said plate, and

(c) exposing said plate to a pattern of activating electromagneticradiation, whereby a latent electrostatic image is produced.

5. A xerographic member comprising an aluminum References Cited UNITEDSTATES PATENTS 3,243,293 3/1966 Stockdale 252-501 NORMAN G. TORCHIN,Primary Examiner J. R. HIGHTOWER, Assistant Examiner US. Cl. X.R. 252--l

